Vapor phase cracking system



Nov. 26, 1940.

A. E. NASH VAPOR PHASE CRACKING SYSTEM Filed Sept. 17, 1958 .AWN

MM j ATTORNEY.

UNITED lSTATES Patented Nov. 26, 1940 PATENT OFFICE "i VAPOR PHASE CRACKING SYSTEM Arthur E. Nash, Mount Airy, Pa., assignor to Petroleum Conversion Corporation, Elizabeth, N. J., a corporation of Delaware Application September 17,1938, Serial No. 230,353

9 Claims. (Cl. 196-60) character.

stream of vapors and in quantity adequate quickly It is a further object of my invention to overcornedisadvantagesor difficulties arising when the charging stock has a Wide boiling range, and this may be true whether it be a light, intermediate or a very heavy oil. With such a charging stock the range of boiling points of the vapors is so great that the higher temperature necessary to convert or crack the vapors of low boiling point is attended by overheating of the vapors of high boiling point, With resultant formation or deposition of carbonaceous material or coke. Otherwise stated, it is an object of my invention to derive from the charging stock a plurality of streams of vapors of different boiling point ranges, and to subject the separate streams of vapors individually to different temperatures orheating conditions best suited to their individual thermal and physical characteristics so that, when mixed with heated heat-carrier gas, they are cracked under optimum time-temperature conditions. f

To this end, and further in accordance With my invention, the liquid oil or charging stock is subjected to temperatures such'as to evolve therefrom vapors of low boiling point range, which are separated from the then Yremaining unvaporized ,A or liquid oil; which residual oil is then subjected to a higher temperature, and within the range producing mild cracking of the residual oil thereby reducing its viscosity by chemically changing its composition; for example by mildly cracking the residual oil, a partial conversion of the heavier into lighter products is obtained. The said lighter products and the material vaporizable at, the higher temperature are then separated to form a second vapor stream. The separately derived vapor streams are individually subjected to heating conditions or temperatures to raise each of them to Within the range of its cracking temperature during time intervals insufficient for material cracking to occur, and before materialcracking has occurred the vapor streams are separately brought into mixture with heat-carrier gas which is itself at a temperature above the cracking temperaturev of the more refractory to raise each stream to a fairly high cracking temperature, for example, totemperature's o f the order of 985 F. to 1100 F. The carrier gas may be brought into mixture with each vaporstream within a plurality of reaction chambers, or equivalents, individual to the several vapor streams; or within the transfer lines yleadingyto the reaction chambers; or in lieu of cracking or completion ofcracking in transfer lines and/or reaction chambers, the carrier gas may ,be'addedl to the vaporvstreams and both subjected'to mild heatingin tubular soakage banksV of a furnace for completion of cracking.

Further in accordance with kmy invention as aforesaid, there'are produced a plurality of,

vapor streams whose vapors are of materially' differing boiling point ranges, these successive vaporizing operations are performed preferablyv in separate zones of different temperatures of a heating unit, preferably comprising a 'plurality of heating orniiring chambers; one or more of the steps of superheating the vapors and heating of the heat-carrier gas may be effected, in one furnace or in separate furnaces;` and one or'm'ore of the steps of superheating the vapors and heating the carrier gas may be effected in the same furnace or furnaces in which one or more of Vthe,

aforesaid vaporization steps are performedfit being desirable in each case to provide relatively independent control of the heat input to, and

the outlet temperatures of, the respective streamsf of fluid undergoing heating. 2,

Further in accordance with my invention, more complete separation of vapors from their attendant liquid oils is accomplished by introducing ash gas into each of the separating Zones. The flash gas is preferably drawn or'tapped off from a stream of heated carrier gas at a point within the heater so that its temperature yis somewhat higher than, or approximately, that of the vapors lustrated a system in accordance with 'my inven' tion, characterized by the provision of a plurality of vapor separators, reaction chambers, and

heaters. Y y 1 With respect to the furnaces herein described',

and their equivalents, for the purposes of my invention, it shall be understood that the tubes in each convection section are heated by convection; the oor tubes are disposed below and without the gas currents to be heated substantially solely by radiation; and the roof tubes are heated by radiation and convection. It shall further be understood that in addition to, or in lieu of one or separate reaction chambers, the carrier gas, and the superheated vapor streams, may, as a mixture of vapors and carrier gas, be subject to further heat in soakage or convection banks of a furnace or furnaces without increase in temperature but to effect soakage and so maintain reaction velocity; or materially all of the cracking of the vapors may be completed in the transfer lines which then carry the separate streams of vapors directly to the scrubber.

Referring to the drawing, the charging stock, y

oil in liquid phase, such as petroleum, topped crude petroleum, or components vof petroleum whose portions to be cracked have suitable boiling point ranges, is introduced into furnace F1, or equivalent heating unit, through the line I0, by pump PI, generally in mixture with recycle stock from the cracking systennas indicated, and under the control of valve I I.

The charging stock is introduced into the heating unit or furnace F1 under suitablepressure such as to maintain upon the entire system, or portions thereof in communication through pressure reducing means, a pressure or pressures, generally superatmospheric, suitable to or usually employed in connection with the particular operating conditions in view; and thepressure in line I is preferably from 250 to 400 pounds per square inch, crit may be more or lessas may be desired, and as determined by the pressure desired upon the processing vessels and devices.

The charging stock passes rst through tubes I2 in the convection zoneor chamber C, disposed between the opposite ends of the furnace or between re chambers I3 and I4, from both of which'the furnace gases pass through chamber C to stack. Into re chambers I3 and I4 hot combustion gases are discharged from combustion muiiles or tunnels I5 and I6, each representing a horizontal series thereof, and respectively provided with burner structures I1 and I8 suited to the fuel to be burned, such as oil, gas, oil and gas; or equivalent, and each separately controllable and adjustable.

From the convection bank I2, the oil traverses the bank 20 of oor tubes, and through transfer line 2| is introducedk into a flashdrum or separator D1 into which is introduced by line 22 ash gas heated in a manner hereinafter to be described to desired temperature, and having the effect in the separator to facilitate sharper or more complete separation of the vapors from the liquid oil by stripping them fromv the liquid and by causing thevapors to have a partial pressure less than the total pressure within'the drum D1. To dry the vapors flash gas is introduced through line 22h before they leave the flash drum.

Further to assistjorto facilitate sharperseparation of the vapors, the separating drum D1 may be provided with several baille plates 23, and reflux ofthermal and physical characteristics about the same as the vapors to be obtained from the separator may be introduced thereinto through line 24, which redux also vaporizes and adds to the supply of dry vapor from separator D1.

The residual liquid oil or bottoms from separator D1 flows through line 25, roof tubes 26` of rire chamber I4, floor tubes 21 and roof tubes 28 of re chamber I3, and thence by line 29 into the second separator or separating drum D2. During passage through tube banks 26-28, the temperature of the liquid is further elevated; and preferably elevated to bring the liquid residual oil to and to hold it at a mild cracking temperature for viscosity-breaking, which effects conversion or cracking of at least a part ofthe heavier stock into lighter boiling point fractions, thereby to increase the yield and amount of Vapor obtainable from separator D2. Flash gas is introduced into separator D2 by way of line 30 to strip the liquid of vapors and to reduce the partial pressure of the vapors. To dry the vapors ash gas is in- `troduced through line 30a as the vapors leave drum D2. Reflux may be introduced through line 3|, further to assist in the separation, and to introduce material which will itself contain vaporizable material to add to the supply of dry vapors issuing from separator D2. The residual liquid oil or bottoms from drum D2 is preferably introduced by a line 33 into scrubber S, for cooling and scrubbing 4the vapors from the reaction chamber. The vapor streams are respectively taken from separators D1 and D2 by way of lines 34 `and 35, to be separately superheated; for example, the vapors from separator D1 are introduced into convection tubes 36 of a heater or furnace F2, and in succession pass through the iloor tubes 31 and roof tubes 38 of the left-hand fire chamber thereof, while the vapor stream of line 35 passes through convection bank 39 and then through floor tube bank 40 and roof tube bank 4I of the right-hand fire chamber. left hand re chambers, as viewed in the drawing, are separately red from their respectivehorizontal series of combustion chambers, muilles or tunnels, I5 and I6, by suitably adjusting the burner structures II and I8 associated therewith, the temperature rise of each vapor streamy may be and is separately controlled. Any desired temperature rise, for given conditions of flow, is readily obtained and in accord withmy invention, each vapor stream is superheated to a temperature-within the range of its cracking temperature, but during a time interval insuffcient'for substantial or material cracking to occur. As shown, the several banks of tubes are connected in series and the tubes of each bank are serially connected. To control the heating time, it is to be understood the tubes within each bank may be connected in groups in parallel with each other; the velocity of flow of the vapor streams may be increased; the tube size increased; and other expediencies known to the art utilized to control the time the vapors remain at their cracking temperature and to maintain that time of such short duration no material cracking of the vapors occurs while undergoing heating, The cracking of the vapors of the separate streams is eected and completed while in mixture with heat carrier gas. The term carrier gas, for brevity, includes gases resulting from a cracking process, including those resulting from the cracking system herein described, hydrocarbon gases, natural gas or steam, and in general, any gas or gaseous heat-vehicle, more particularly if it be of character which does not adversely aifect the process or desired product by polymerization, chemical or other action.

The heat-carrier gas is introduced into av furnace F3 through a supply line 42 leading from a gas compressor P2 under such pressure as will cause it to have a pressure equal to or .suitably related to the pressures of the vapor streams..

Since the right and .azzaesa 'way or `une e2 into the bubble tower or frac- The carrier gas passes first'through-a bank of convection tubes d3, thence through oor tubes fili and roof tubes i5 ofthe lefthand fire chamber, thence throughgroof tubes `I6 and floor .tubes 4l' of the right hand fire chamber, and by transfer line 'i8 is introducedintollines 49v and 50,- respectivelyincluding valves 5l and 52.' I i The temperature of the heat-carrier gasis raised above that of the'more refractory vapor streams, the one from separator Dl which passes through floor tubes 31 and roof tubes-38 and by transfer line 53 to reaction chamberRl, and may be of the orderof fromv1200 F. to 1300 F.

Instead of a 'separate bank of tubes Within one of the heaters FI, F2 or F3, I prefer to provide a line 545 connected into the heating tubes of heater F3 at that point where the heat-carrier gas is at a temperatureissuitable for withdrawal and supply to the lseparators D1 and D2.. As shown, the fiash gas, withdrawn from the heatcarrier gas after it has passed through floor tubes 'dfi and roof tubes 45, under the control of valves 54a and 5th passes into the separators D1 and D2. Within f the transfer line- 53 the heat-carrier gas `is 'commingled with the more refractory` lstreamof vapors which have been superheated to a temperature between I945 F. to'1065fF. The cracking reaction is immediately -initiated and rapidly proceeds to completion; it may be substantially complete before thevap-ors in mixture with carrier gas reach the reactionchamber R1, or the cracking reaction maybe completed within the reaction chamber R1.

Similarly and concurrently, the lcarrier gas supplied by line 50 is in transfer line 55 commingled with the less refractory vapor stream from roof tubes il and separator D2. The cracking reaction of the less refractory vapors, which have been superheated to about 930 F. Ito 975 F., isy immediately initiatedand may becompleted within the transfer line 55, or the final stages of the cracking reaction may be completed Within the second reaction" chamber R2. In each case the temperature of v'themixture is the optimum for the thermalv characteristics of the vapor stream, and by varying` thetemperature of the superheated vapors, theamount of carrier gas or its temperature, temperatures for the mixtures are readily attained which will produce maximum formationof gasoline.

The reaction chambers R1 yand R2 discharge the reaction products .and gas into the scrubber S, from whose bottom is drawn off through lines 56 and 5I oil which is unsuitable for zrecycling through the system, and this may be utilized as fuel oil, placed in storage, or further processed for the production'of asphalt 'and the like. Preferabiy, a part of the liquid oil, the division being determined by valves 51a and 59a from the bottom of the scrubber S is passed through a cooler 58, elevated in pressure by pump P3, and introduced 'into the scrubber by line 59 in the form of a spray thoroughly to contact and cool and scrub the reaction products within the scrubber. Additional cooling and scrubbing mediums, or reflux, may be introduced into the scrubber which contains a few trays or bailies (not shown); as for example, a stream `of heavy material may be derived from bubble tower B, elevated in pressure by pump P4,v and by line 50 introduced into the upper part of the scrubber, anddescends countercurrently with the ascending vapors and gas; and a stream from the bottom of separator D2 may be introduced by Way of line 33., v l

The cooled vapors from thefscrubberpass by.

Valve Il, recycled in mixturewith the fresh feed or charge stock'.

. From thev bubble tower B the desired reaction products and gas are discharged through a cooler or condenser C, in heat ltransfer relation with a suitable cooling medium entering, for example,

condenser C the gas andliquid products are discharged intoa gas separator and gasoline accumulator A, from which the gas is discharged through lineand to` the gas compressor P2, or a part thereof, under control of valve 16, is sent to storage, or burned asfuel.

The liquid product, such as motor fuel, including gasoline, is drawn off through line 1|. A part of the liquid product, under the control of valve I2, is elevated inpressure'by pump P6, and by line 13 introducedy into the bubble tower as reux therefor. The remainder of the liquid product under theV control of valve M is drawn off to storage or otherv destination, and preferably -formsthe supply for a stabilizer, or for further 110 .at 6'! and discharged at 68. From the cooler or conditions. under which the cracking o'r other steps are 'effectedff `Where reducing valve or Valves is or are used at one or'more points along the system for stepping down the pressure, the pressure backed up by valves 74 and 15 deter# mines the pressure on the low-pressure side of the last reducing valve.

The methods and apparatus described provide greater yield of desired product, motorfuel and especially gasoline when" producing a product `of a given octane value or anti-knock characteristic, and procure a greater yield of a desired product for a system of l given size. In kaccord with my invention less stock is recycled, which is to say, the amount orlrate of conversion or cracking per pass or single traverse of the system is to an unusual degree increased. Y y

By Way'of example, the particular charging stock, fresh feed, such as stripped or topped crude'petroleum, may". have an initial boiling point of about 350 F., and a gravity within the range o-f 19 A.l P. I. to r25" A. P. I. Such stock which 'ordinarily is combined with recycle stock, is heated, vaporized and separatedas described into the two streams of vapors, the first from separator D1, representing or comprising low boiling oil having an initial boiling point of about- 350 and a gravity within the range between A. P. I. and 35 A. P. I. and comprising about to 50% of the charging stock, while the vapors of the second stream from separator D2, comprise or represent a heavier oil, forexample having an initial boiling point of about 550 and a gravity within the range between 15 to 25 A. P. I. and comprising about to 70% of the residual oil from separator D1, the second stream including lighter fractions resulting from the mild'cracking or viscosityebreaking incidentk to the heating of the residual liquid oil from the separator D1, before its admission. to separator D2. The residual liquid oil from separator D2 having a gravity Within the-range of 5 to 15 A. P. I is'through line 33 introduced into the scrubber.

The light .or more refractory vapors of lower boiling 'oil are, for example, then superheated to cooling medium in countercurrent contact with Ya temperature between 945 F. and 1o65 F., and

the vapors of the heavier, less refractory stream are superheated to a temperature of the order of 930 F. to 975 F. Before material cracking of vapors has occurred, the carrier gas is introduced into the transfer lines 53 and 55 and, commingling with them, quickly elevates the vapors to a higher temperature, and produces immediate and complete cracking thereof. The cracking reaction is ordinarily completed in a very short time interval, for example, within an interval between about one-half a minute and two minutes. The carrier gas is preferably heated to a temperature between about 1150 F. and 1300 F., and to a temperature such that, when the carrier gas has been added to the vapors, the temperature of the mixture will then be best suited to the particular thermal and physical characteristics of each stream` for production of the maximum amount of gasoline. For optimum conditions for the example here considered, the temperature of the mixture of the carrier gas and of the lighter, more refractory-vapor stream may be between about 985 F. and 1100 F., and the `temperature of the heavy, less refractory stream `between about 950 F. and 1075" F., in part depending upon the octane value desired in the product, ordinarily high, as for example from 68 to 75. i

After passage through the reaction chambers `Rl and R2 the reaction products and gas, upon entering the scrubber S are thoroughly contacted by a cooling medium which scrubs and cleans and condenses from the reaction products heavier materials contained therein. A part of the cooling medium, derived from the outlet 56 from the bottom of the scrubber S, after passage through the cooler 58 is by pump P3 forced into line 59 which terminates within the scrubber in a spray head or nozzle (not shown), which sprays the the reaction products and vapors.

Further to increase the yield of desirable products, it is desirable to take a side stream from the bubble tower B by way of line 24, the side stream having physical characteristics, particularly a boiling range substantially corresponding with that of the vapor stream taken fromthe separator D1. Included in the line 24 is a pump PI to elevate the side stream to a pressure suflicient for entry into the separator D1. The side stream 24 serves as a temperature control for the separator D1, and aids in securing a sharper separation of the desired vapors from the charging stock. Substantially all of the side stream, recycle or reflux, is vaporized. Similarly, a part of the bottoms from the bubble tower may be introduced into the second separator D2 by way of lines 64 and 80, valve 8l, and line 3| to control the temperature at the top thereof, and to comprise reux therefor. By introducing both of the stocks or streams aforesaid directly into the separators, the quantity of vapors in the respective vapor streams 34 and 35 may be increased without corresponding increase in the charge to convection bank I2, which makes for increased throughput, less demand on the heater F1 and in general increased yield of desirable products.

By the method and apparatus illustrated, the vapors of the individual streams are more homogeneous than in the case Where a single stream of vapors is derived from the charging stock, especially when of wide range of boiling point, including relatively low and high boiling components. The vapors of each of the streams are 'cracked or treated at a temperature and under attendant conditions better suited to their particular physical and thermal characteristics. Maximum cracking or conversion is procured without overcracking or coking of the heavier components of the charging stock to be cracked, and without undercracking or incomplete conversion of its lighter or lower boiling components.

Employment of heating units or furnaces of the character and for the purposes herein described greatly increases the economy or eficiency of operation; the control of application of heat for any step of the process or any part of the system is improved; and the several controls of heat application with respect to each other yields an overall or inclusive control of improved flexibility, from which follow greater efficiency of operation, improvement in or control of characteristics of product, and with respect to the carrier gas, a continuous ow thereof, controllable as to its rate of ow in comparison to the rate of flow of the vapors to be cracked, and with delivery of heat to the vapors and to the reaction chamber, or equivalent, at a constant and controllable rate per unit of time, as distinguished from the operation of stoves and particularly regenerative furnaces of which it is characteristic that there is direct heat transfer from hot refractories to the carrier or cycle gas instead of the indirect heat transfer thereto through the tubes of the heaters or furnaces in which fuel is continuously burned to generate heat and combustion gases. In the stoves the carrier gas fluctuates in temperature, and therefore, requires compensating fluctuation in rate of iiow in an endeavor to procure heat input into the reaction chamber at substantially constant rate per unit of time. Whereas, in accord with my invention the carrier gas is heated in a continuously flowing stream by passing it through a furnace in indirect heat-transfer with combustion gases, whose heat is indirectly transferred to the carrier gas through the walls of the tubes; the tubes themselves, depending on their location' and position with respect to the currents of gases as already explained, receiving heat substantially solely by radiation, and by combined radiation and convection from the combustion gases.

While I have shown a particular embodiment of my invention, it will be understood that I do not limit myself thereto, since many modications may be made, and I therefore contemplate by the appended claims to cover any such modiiicati-ons as fall within the spirit and scope of my invention.

What I claim is:

1. The method of cracking hydrocarbon oil in vapor phase which comprises heating the oil in liquid phase in a restricted stream to vaporization temperature, separating therefrom a first overhead stream of vapors, further heating the residual oil in liquid phase to a higher vaporization temperature, separating therefrom a second overhead stream of vapors of boiling point higher than that of the vapors of said rst stream, separately superheating the vapor streams by indirect heat-transfer from combustion gases, the rst stream to a temperature substantially above the temperature of said second stream, heating a continuously flowing stream of heat-carrier gas above the cracking temperature of said rst stream of vapors by passing it through a furnace in indirect heat-transfer with combustion gases, and commingling said carrier gas with each ol said separately superheated vapor streams in aaaacea quantityL adequate to elevate each said streamV to a temperature best suited to the thermal and physical characteristics of the vapors thereof for maximum cracking of each vapor stream'into products Asuitable for motor fuel.

liquid phase in a restricted stream to vaporization temperature, separating therefrom a first overhead stream of vapors, further heating the residual oil in liquid phase to a temperature producing vmild cracking thereof and viscocitybreaking, separating therefrom a second overhead 'stream of vapors of boiling point somewhat ihigher than that of the vapors of said first separately superheating said vapor streams to temperatures Within the range of their Stream,

said cracking temperatures during time intervals insucient for material cracking to occur, heating a continuously flowing stream of heat-carrier gas above the cracking temperature of said rst stream of vapors by passing it througha furnace in' indirect heat-transfer with combustionl gases, and commingling said carrier gas With each of 'said separately superheated vapor streams in quantity to elevate the respective vapor streams to temperatures -best suited to them for the production of products suitable as motor fuel.

3; The method of cracking hydrocarbon oil `in vapor phase which comprises heating charge oil in` liquid phase in a restricted stream to a temperature adequate -to produce substantial vaporization thereof, in a zone of reduced pressure separating the oil into an overhead stream of vapors and a stream of residual oil, further heating the-residual oil in liquid phase to a ternperature adequate to viscosity-break the residual oil, separating said residual oil into a second overyheadvapor stream and into a second stream ci 'residual oil, separately superheating the vapor streams tovtemperatures respectively Within the cracking-temperatures of vapors thereof during time intervals insumcient for material cracking to occur, in a furnace burning fuel to generate *Shot -combustion gases, heating a continuously flowing stream of heat-carrier gas vabove the cracking temperature of said rst stream by passing it through said furnace indirect heattransfer with said combustion'gases,v in reaction ifi'zones lcommingling said carrier gas with each of said separately superheated vapor'streams quickly to elevate each of them to temperatures best suited to the vapors of said streams for the production of products suitable as motor fuel, in a scrubbing zone receiving the cracked vapo-rs from said reaction Zones, applying cooling oil to said cracked vapors, withdrawing oil in liquid phase from the scrubbing zone, and after cooling it reintroducing at least a part of it into said scrubbing zone as said cooling oil.

4. The method of cracking hydrocarbon oil in vapor phase which comprises heating the oil in liquid phase in a restricted stream to a temperature Within the range of its vaporization temperature, in a zone of reduced pressure separating the oil into an overhead stream of vapors and a stream of residual liquid o-il, further' heating said residual oil to a temperature sufficiently high mildly to crack said residual oil so that a substantial portion thereof will vapcrize in a separating Zone having a pressure substantially the same as that in said first separating zone, in

of said firstl stream,l separately superheating the vapor streams, the rst-named stream to a temperature' substantially above the temperature of said second stream, in `a` furnace heating a continuously flowing stream yof heat-carrier gas above the cracking temperature of said firstnamed stream of vapors by indirect heat exchange with combustion gases generated YWithin said furnace, and commingling said carrier gas with each of -s'aid separately superheated vapor streams in quantityto elevate the temperatures to those best suited to their thermal and physical characteristics forv the production of products suitable'for motor fuel of high octane number.

5. Ak system of cracking hydrocarbon oil comprisinga plurality of vapor separators respectively provided With an'inlet, an overhead vapor outlet and a liquid oil outlet, means including a furnace and `awilrst bank of tubes therein for connecting the liquid oil outlet of one vapor separator to the inlet of a second vapor separator, means for conducting oil, at vaporization temperature to the first of said separators, the liquid oil therefrom vpassing through said first bank of tubes and into said inlet of said second separator aftermaterial elevation of its temperature, means formingreaction zones, means including a furnace yand banks of tubes therein individual to said separators and respectively Within separate re chambers for separately conducting Vthe vapors from said lseparators to said reaction zones, said vapors during passage through their respective banksof tubes receiving heat to elevate them to temperatures Within the range of their respective cracking temperatures, means including a furnace and at least one bankof tubesl for heating a heat-carriergas above the discharge temperature of anyof said' separate streams of said oil vapors, and means'for commingling Within said reaction lzones said carrier gas and said separately superheated "oil ,vapors lto produce simultaneous substantial cracking of said oil vapors at temperatures best suited to said vapors. i

6. Asystem of cracking hydrocarbon oil in vapor phase comprising a vapor separator having a liquid inlet, `an'overhead vapor outlet and a liquid outlet, means connectedto said liquid inlet for conducting oil at vaporization temperature to said separator, tubular heat-absorbing means connectedA to,v said" lv a'p'or outlet for su'perheating the oil vapors tov at least their cracking temperature during a time interval insufficient for material cracking thereof to occur Within said tubular means, a second vapor separator having a liquid inlet an overheadvapor outlet and a liquid outlet, means for conducting oil in liquid phase from said liquid outlet of said rst-named separator to said liquid inlet of said second separator and including a second tubular heat-absorption structure for further elevating vthe temperature of said oil in liquid phase, means connectedto said vapor outlet of said second separator and including a third tubular heat-absorbing structure for superheating said vapors to at least their cracking temerature during a time interval insufficient for material cracking thereof to occur Within said .third structure, means forming reaction zones, means for heating a heat-carrier gas above the cracking temperature of the more refractory of said oil vapors comprising a fourth tubular heatabsorption structure, means for .deriving from said fourth structure gas at approximately the temperature of the liquids entering said separators and for delivering said gas to said separators to assist vaporization of oil therein, and means for commingling the carrier gas Within said reaction zones with each of said separately superheated vapors to effect substantial cracking thereof at temperatures best suited to the individual vapor streams.

7. The method of cracking hydrocarbon oil in vapor phase which comprises heating charge oil in liquid phase in a restricted stream to a temperature adequate to produce substantial vaporization thereof, in a zone of reduced pressure separating the heated oil into a rst overhead stream of vapors and a rst stream of residual oil, further heating the residual oil in liquid phase to a temperature adequate to viscosity-break the residual oil, in a second zone separating said viscosity-broken oil into a second overhead stream of vapors of boiling point higher than that of the vapors of said first stream and a second stream of residual oil of low A. P. I. gravity, separately superheating the vapor streams to temperatures respectively Within the ranges of cracking temperatures of the vapors thereof during time intervals insuflicient for material cracking to occur, heating a continuously iiowing stream of heatcarrier gas above the cracking temperature of said rst vapor stream by passing it through a furnace in indirect heat exchange with combustion gases, in reaction zones commingling said carrier gas with each of said separately superheated vapor streams quickly to elevate each of them to a temperature best suited to the vapors thereof for the production of products suitable as motor fuel and to maintain them at said elevated temperatures until completion of the crack- ,ing reaction, immediately thereafter introducing said vapors and gas into a scrubbing zone, and in said scrubbing zone intimately spraying said vapors and said gas with a scrubbing and cooling medium comprising at least in part said second stream of residual oil.

8. The method of cracking hydrocarbon oil in vapor phase which comprises heating the oil in liquid phase to raise it to vaporization temperature, in a rst zone separating therefrom an overhead stream of vapors, further heating the residual oil to raise it to a higher temperature of vaporization, separating therefrom 'a second overhead stream of vapors and a second stream of residual oil, said second vapor stream having a boiling point higher than that of said rst-named vapor stream, separately superheating said streams of vapors, the rst-named stream to a materially higher temperature than said second stream, heating heat-carrier gas to a temperature above the temperature of said first-named vapor stream by passing it through a furnace in indirect heat-transfer with combustion gases,y in reaction zones commingling said carrier gas with said superheated vapor streams to supply heat to effect substantial cracking of said vapors at temperatures best suited to the individual streams thereof, after completion of the cracking of said vapors intimately contacting them with a cooling and scrubbing oil at least in part comprising said second residual oil stream, thereafter fractionating the reaction products to produce an overhead stream of products suitable as motor fuel and a side stream of characteristics similar to the vapors leaving said rst separating zone, and introducing said side stream into said first separating zone as reflux therefor.

9. A system of cracking hydrocarbon oil coni-r prising heating units, each having separate fire chambers and a common convection chamber, means individual to said :lire chambers for producing therein currents of hot combustion gases which pass into and through their associated con- Vection chamber, a first bank of tubes within one of said convection chambers, means for conducting oil through said first bank of tubes, a separator having an inlet receiving the oil from said rst bank of tubes and an overhead vapor outlet, means for conducting flash gas to said separator including a second bank of heat absorbing tubes within one of said re chambers for elevating the temperature of said flash gas passing to said separator, means including a third bank of tubes Within a second of said fire chambers for heating the oil vapors from said separator to a temperature within the range of their cracking tempera- 

